1. Field of the Invention
The present invention relates to a nonlinear optical silica material based on SiO2xe2x80x94GeO2, and more particularly to a nonlinear optical silica material having nonlinear optical properties which are exhibited readily, and which is very reliable. The invention also relates to an optical functional device using the nonlinear optical silica material and a method of producing it.
2. Description of the Related Art
It is almost inevitable that the field of optical communications will be the subject of major development in the future. A light-controlled switching device, a wavelength conversion device, an optical functional device such as an optical memory and an optical sensor are major components in an optical communication system. An optical material having nonlinear properties is also important to produce such optical functional devices.
As a nonlinear optical material, a crystalline material such as LiNbO3 (lithium niobate) has been put to practical use.
However, when this nonlinear optical material LiNbO3 is used as an optical device, its physical properties are very different from those of glass (e.g., optical glass fiber) used as a connecting member, and this difference in physical properties has a problem of causing a defect in the optical device. Also, LiNbO3 is bulky and quite difficult to make thin film. Such a bulky optical functional part and a semiconductor device must be produced separately and then combined together, resulting in a disadvantage in view of production costs. Besides, it is difficult to form the bulky nonlinear optical device minutely, so that it can not easily be made highly functional.
Under such circumstances, SiO2xe2x80x94GeO2 glass is now under development as a nonlinear material which can be formed into a thin film and is based on silicon glass having only slight differences in physical properties from silica glass or the like. FIG. 1 is a schematic diagram showing a three-dimensionally configured bonded state of the SiO2xe2x80x94GeO2 glass in a two-dimensional form. This SiO2xe2x80x94GeO2 has less optical loss, a wider transmitted wavelength area and better processability and durability compared with a conventional material. Also, SiO2xe2x80x94GeO2 material is considered to be readily formable into a thin film, and if the nonlinear optical material, SiO2xe2x80x94GeO2, can be formed into a thin film, it can be readily made into a hybrid with a semiconductor (electric) device such as an IC and LSI. It is well known that ICs and LSIs are produced by performing heat treatment and thin film formation while finely processing the surface of a substrate of Si, GaAs or the like. Therefore, a process of forming the nonlinear optical material can be incorporated into the process of manufacturing an IC or LSI using the SiO2xe2x80x94GeO2 material formed into a thin film, so that a optical semiconductor hybrid element can be produced. In addition, development into a high-integrated optical device (optical IC) is also high. SiO2xe2x80x94GeO2 has a material composition similar to those of a main semiconductor material Si and an oxide film (SiO2) which is formed when Si is processed and also has good applicability to a semiconductor production process compared with a conventional nonlinear optical material.
However, this proposed SiO2xe2x80x94GeO2 film which can be made thin is still in a research stage and satisfactory nonlinear optical properties have not been obtained yet. Neither are its element composition and manufacturing method optimized, and so it has not been put to practical use. In order to impart nonlinear optical properties to this SiO2xe2x80x94GeO2 film, its poling must be performed by applying a high electric field of about 105V/cm. However, a high electric field causes breakage of the semiconductor element or degradation in performance when a hybrid element is configured by incorporating an IC, LSI or the like having other circuit elements (transistors, capacitors and the like) on a semiconductor substrate such as Si which is also used for the nonlinear optical material film. There is therefore a demand for satisfactory nonlinear optical properties to be obtainable by applying a low electric field so that an adverse effect owing to the application of the electric field to the SiO2xe2x80x94GeO2 film is not applied to other semiconductor elements.
When the SiO2xe2x80x94GeO2 film is formed on a semiconductor substrate of Si or the like, elements such as Ge of the SiO2xe2x80x94GeO2 film are dispersed in the semiconductor substrate (Si), causing a breakage in the semiconductor substrate and also the transistors, capacitors and other elements formed on the semiconductor substrate or degradation in performance thereof. Furthermore, there is likely to be a problem that when the SiO2xe2x80x94GeO2 film is formed on the semiconductor substrate, a defect in the crystal of the surface of the semiconductor substrate is increased.
It is an object of the present invention to provide a material which can exhibit satisfactory high nonlinear optical properties by applying a weak electric field which does not affect other elements even when an SiO2xe2x80x94GeO2 film having physical properties somewhat different from silica glass is used as an optical material.
It is another object of the invention to lower an adverse effect on the other elements caused by the presence of a nonlinear optical silica film mainly consisting of SiO2xe2x80x94GeO2 formed on a substrate such as an Si substrate which has other semiconductor elements formed thereon.
The present invention has been achieved to complete the aforesaid objects and has the following features.
First, the nonlinear optical silica material of the invention has Sio2xe2x80x94GeO2 as a main component and is characterized by having a hydrogen or halogen element added to this silica material.
The invention is also characterized in that oxygen, which is bonded to Ge contained in the nonlinear optical silica material mainly consisting of SiO2xe2x80x94GeO2, is substituted by hydrogen.
The invention is also characterized in that oxygen bonded to Ge contained in the material is substituted by elemental halogen to totally or partly take the place of hydrogen.
In the nonlinear optical material mainly consisting of SiO2xe2x80x94GeO2, Si and Ge elements having four coordinates are bonded through O elements to form Sixe2x80x94Oxe2x80x94Si, Sixe2x80x94Oxe2x80x94Ge and Gexe2x80x94Oxe2x80x94Ge bonds. Among them, a part where the material exhibits nonlinearity is a part where all valence arms of Ge elements having four coordinates of the Gexe2x80x94O bond do not bond with O elements but some valence arms have remained as nonvalence arms, namely a so-called dangling bond (unpaired electron) is present.
The nonlinear optical silica material according to the present invention has, for example, a structure indicated in a two-dimensional form as shown in FIG. 2. It is seen in FIG. 2 that H (hydrogen) is added to the material so to have Sixe2x80x94H and Gexe2x80x94H bonds (Sixe2x80x94X and Gexe2x80x94X bonds when halogen element X is added) in the material film. In the bonded state of points (Gexc2x7) where nonlinearity is exhibited, the material of the present invention (see FIG. 2) has two Gexe2x80x94O bonds and one Gexe2x80x94H (Gexe2x80x94X when halogen element X is added) bond with respect to one Ge while a conventional material (see FIG. 1) has three Gexe2x80x94O bonds.
Thus, Gexe2x80x94H bonds and Gexe2x80x94X bonds which do not relate to bonding of crystal networks can be formed at points (Gexc2x7)where nonlinearity is exhibited by adding hydrogen and halogen elements to the nonlinear optical silica material. In other words, since the Gexc2x7 points have a bond not related to the crystal network, the electric field applied can be lowered substantially compared with a conventional nonlinear optical silica material in processing for polarity orientation (poling) to cause the nonlinear optical silica material to exhibit nonlinearity.
As described above, the nonlinear optical silica material according to the present invention can be used as a thin film on a semiconductor substrate so to form, for example, a optical semiconductor hybrid element having a silicon semiconductor element on the same silicon substrate. When such a hybrid element is produced, the semiconductor element is prevented from being broken or degraded in performance by an electric field applied for orientation of the polarity of the nonlinear optical silica material. Therefore, fine processing and formation of a multifunctional element can be performed simultaneously by forming the nonlinear optical silica material of the invention into a thin film and incorporating it into a semiconductor element, and the functions of the optical device can be improved remarkably, and significant reduction in costs can be made.
Thus, by adding hydrogen and halogen, flexibility of arrangement of the elements in the material can be enhanced, stress applied to the film is decreased, and the separation of the film from the substrate can be prevented. Therefore, a defective rate in forming the optical semiconductor hybrid element by combining the nonlinear optical film and the semiconductor element can be lowered. The reliability of the element can also be improved in view of the above-described points.
Furthermore, a defect inherent in the material is decreased by virtue of hydrogen and halogen, and reliability and stability are also improved.
In this invention, H or halogen element X is suitably added in an amount to fully exhibit an effect of its addition without disturbing the basic SiO2xe2x80x94GeO2 bond. A composition ratio of the Si and Ge elements is at a level to fully secure a necessary optical transmission factor required as the optical material and to fully exhibit a nonlinear effect by Gexc2x7. Oxygen is desired to be contained in an amount necessary to obtain Sixe2x80x94Oxe2x80x94Si, Sixe2x80x94Oxe2x80x94Ge and Gexe2x80x94Oxe2x80x94Ge bonds.
The nonlinear optical device according to the invention is characterized in that the aforesaid hydrogen and halogen are contained in the nonlinear optical silica material, or the nonlinear optical silica material having oxygen bonded to Ge substituted by hydrogen and halogen is formed on a desired substrate, and an insulating thin film is intervened between the nonlinear optical silica material and the substrate.
Such an insulating thin film, which is interposed between the nonlinear optical silica film and a desired substrate, has functions to prevent a defect from increasing on the surface of the substrate and the Ge elements from dispersing into the semiconductor substrate when the silica film is formed on the surface of the substrate.
Therefore, dispersion of undesired impurities into the semiconductor substrate and other elements such as transistors and capacitors which are important as the semiconductor elements or occurrence of a defect in the substrate can be prevented, and a functional element excelling in reliability can be produced.
The insulating film can be made of a silicon compound such as silicon oxide (SiOx) or silicon nitride (SiNx). The insulating film has a thickness of 0.1 to 10000 nm, and more preferably 2 to 1000 nm.